1. Field of the Invention
The present invention relates to an image reader for a stationary original placed on a platen glass irradiated with light beams emitted from a light source lamp for reading an image recorded on the original on the basis of light beams reflected from the original and particularly to a mechanism for positioning the platen glass of such an image reader.
2. Description of the Related Art
An image reader such as copying machine or scanner adapted to read an original image data recorded on paper sheet or the like for subsequent various kinds of processing is generally constructed so that the original is irradiated with light beams emitted from a light source lamp such as fluorescent lamp. Light beams reflected on the original are guided to enter a light receiving station provided with a photoelectric converter device such as CCD (Charge-Coupled Device). In the case of the image reader for a stationary original, the original is placed on the platen glass and is irradiated with light beams emitted from said light source lamp which is moved relative to said original. Not only the light source lamp but also a reflector and the other components adapted to guide the light beams reflected on the original to the light receiving station must be moved along the original in order to maintain a optical path length from the original to the light receiving station even when the position at which the original is irradiated with the light beams varies.
The position from which the light source lamp, the reflector and the other components start to move is constant independent of various factors including the size of the original. Unless the original placed on the platen glass is set to a predetermined position, the position from which the image recorded on said original starts to be read will not coincide with the end position of the image and it may become impossible for the image reader to read the image recorded on the entire area of the original. If the original is oriented out of alignment with the predetermined direction in which the light source lamp, the reflector and the other components move, a correspondingly misaligned image will be picked up. In order to ensure that the image can be read starting from the end of the original and the original is properly aligned with said predetermined direction, it is essential to set the original to the predetermined position on the platen glass.
Conventionally, a structure provided with a guide member against which an end of the original is placed as the original is placed on the platen glass has been used for positioning of the original and such positioning guide for the original is described, for example, in Japanese Laid-Open Patent Application Gazette No. 2000-258857. The positioning guide for the original comprises a narrow plate-like member having a datum edge against which the end of the original is placed and a mounting projection formed along the other edge of said narrow plate-like member. The mounting projection is formed with threaded holes, U-shaped notches and tapered regions, on one hand, and a top opening of the image reader's frame is formed along its peripheral edge with threaded holes in alignment with the threaded holes of said mounting piece and projections adapted to seat in the respective U-shaped notches, on the other hand. The tapered regions communicate with the U-shaped depressions and have inner diameters gradually enlarged as these tapered regions go away from the U-shaped depressions. The platen glass (contact glass) on which the original is placed is mounted on the image reader so that one end of said platen glass overlaps said datum edge of said narrow plate-like member.
When the image recorded on the original is read by the image reader provided with such positioning guide for the original, the original is set by placing its one end against the datum edge of said narrow plate-like member.
With the positioning guide for the original described in the above-cited Japanese Laid-Open Patent Application Gazette No. 2000-258857, the projections formed on the image reader's frame are engaged in the U-shaped notches to achieve positioning of said narrow plate-like member. However, these projections are formed on the image reader's frame and therefore may raise problems as follow. As has previously been described above, the position to which the original is set depends on the position from which the light source lamp, the reflector and the other components start to move. In order to set this position from which the components such as the light source lamp and the reflector start to move at high accuracy, the image reader's frame and the other components must be installed at correspondingly high precision. In consequence, the cost for installing them correspondingly increases. To avoid this, the end of the original as well as the position from which said components start to move has practically been adjusted basically by adjusting the position to which the original should be set. In other words, such positioning guide of the original of prior art has usually made a certain allowance for the setting accuracy of the position from which said components should start to move. In order that such adjustment can be effectively carried out, a certain play must be defined between said projections of the image reader's frame and the associated U-shaped notches. As a result, when the platen glass and the positioning guide for the original having been demounted once are installed again, the position at which said positioning guide of the original is fixed must be adjusted again with much labor and time.